Sorry to hear about your problem. Based on the information you've given, we may have to do a little more work to come up with a solution. It is not my opinion that the pubescence or the pH are working against you here. The pubescence on the midrib pictured is perfectly normal physiological characteristic of many Prunus species (and many other tree species for that matter), and nothing to be alarmed about. If you've noticed that many of the fallen leaves have this pubescence, it is an indication that more mature, older growth has fallen, which could be a hint pointing us in the right direction.
Secondly, a pH of 5.0 is definitely on the lower end of the range in which this tree will thrive, but it's not typically low enough to facilitate harm to the tree. Prunus cerasifera is well known as adaptable to lower than usual pH's, and generally a pH of 5.0 is considered okay for planting and establishment as long as fertility, soil structure, moisture levels and drainage are all adequate as well.
So, why else may the leaves be dropping? Prunus cerasifera typically has a life of about 20 years before decline and death, so the ages you mentioned shouldn't be a factor here. You mentioned that this is the second year this has happened. How severe was the defoliation last year? Did the trees lose all of their leaves? Half? A quarter? Also, have you had a soil test to determine nutrient levels/needs?
Given the time of year and the seemingly healthy look of the trees based on the pictures, there is a possibility that the leaf drop you see is just a natural process that many trees go through around mid-summer. It's quite normal in the spring time for a healthy tree to produce more foliage than the root system can support, and once those dog days of mid-summer come along, you'll see the tree shed some of those older leaves and ease back on producing new growth. The reality is that around here, experiencing numerous week-long heatwaves throughout summer with temperatures regularly rising to 100 degrees F and no rain is the new norm, and it goes without saying that that will absolutely cause stress reactions in trees like leaf-drop.
In your case, if the leaves that have dropped look free of spots or deformities, and they are dropping relatively uniformly throughout the tree as opposed to dropping from specific, concentrated sections of the canopy, and when it's all said and done only a small percentage of the overall canopy has dropped (like no more then 20%), run-of-the-mill summer leaf drop would be my best guess based on the information I have now. If this were the case, you would want to make sure that the tree is getting watered regularly this time of year and that the soil around the trunk isn't too dry and hard, and that you have mulch down below the tree to help regulate moisture loss. Additionally, making sure that the mulch at the base of the tree is piled no higher up the trunk than where the roots flare out from the trunk is always critical to avoid these and other problems.
Now, what I see in your pictures are healthy, full canopies with good color, that look exactly like healthy canopies should look. However, another thing that Prunus cerasifera is well-known for is being susceptible to all manner of diseases and insects under the sun, so we can't assume anything just yet. There is a lot more to potentially rule out here, and a closer look at the following is needed to do so.
Aside from the pubescence, do you see any spots, deformities, discolorations or otherwise unusual characteristics of the leaves still on the tree or on those which have fallen? Do you see evidence of insects on the leaves or leaf stems, such as scale, aphids, mites, or the very small, pin-size speckling on the leaves that would indicate feeding of those insects? Do you see sooty mold growing on the leaves, stems, branches, trunk or anywhere under the tree canopy? Sooty mold is a black mold that grows exceedingly on the excrement of many plant feeding insects. Do you see any heavy insect webbing throughout the canopy of the trees? Do the leaves look chewed, or do you see leaf-chewing caterpillars or Japanese beetles at work? If you see any of this, could you describe it and send pictures?
Secondly, could you closely inspect the trunk and the branches to look for any sign of rot, mold, fungus or bacteria? This would manifest itself as a wound or damage on the bark that doesn't match the general form, texture or look of the bark throughout the tree. In extreme cases it could be dark, water-soaked or wet-looking bark that may even be oozing sap or a sap-like substance.
Thirdly, could you dig around in the soil at the base of the tree to determine the moisture level of the soil? Is it constantly soaking wet even when it hasn't rained? Is there a layer of mulch thicker than 2-3 inches? Is the ground very dry and hard/compacted? Is it somewhere in the middle between very wet and very dry? If you see this, could you describe it and send pictures?
Needless to say, this is a lot to do to assess a tree. If you are concerned and feel you need real-time, in-person assistance to make an evaluation with which you are comfortable, please contact the Norfolk Cooperative Extension Office at 757-683-2816.